The present invention relates to an improved mold and a method of making molds for large scale equipment shelters or the like.
In locations which are subject to extreme environmental conditions such as the north slope of Alaska or the equatorial tropics, it is common for delicate electrical and electronic equipment to be housed within protective shelters. Such equipment can include electrical and telephone switching panels, circuit breakers, and electronic controls for pipelines, aircraft guidance equipment at airports, etc. The shelters must be constructed to withstand physical shocks as well as exposure to extreme temperature and moisture conditions. It is known to construct such shelters from unitary inner and outer shells of impermeable polymeric material which are bonded together by a layer of rigid polymeric foam sandwiched therebetween. Normally, the inner and outer shells are separately molded, the inner shell is then inserted into the outer shell, and the polymeric foam is injected between the shells and allowed to harden. The resultant structure has excellent structural strength and insulating qualities.
Separate molds are required for both the inner and the outer shells, and the shells are normally constructed by laying up or spraying a glass reinforced plastic, such as Fiberglas or the like, or other composite material on the mold which is then allowed to set and is removed to form the finished shell. In the case of the outer shell, this requires a worker to apply the composite from within the mold, resulting in a cramped and dangerous condition since it is difficult to properly ventilate such a small space. Recognizing this problem, the present inventor has created a two-part mold for the outer shell so that, during molding, one end of each mold part is open. The outer shell halves molded therefrom are subsequently assembled into a unitary outer shell structure. This invention is shown and described in applicant's U.S. Pat. No. 4,156,998, patented on Jun. 5, 1979, which is incorporated herein by reference.
In creating the shell molds, including the two-part mold of the above-cited patent, it has been traditional to first create a full scale model or pattern of each shell to be molded. A laminate is applied onto the pattern and allowed to cure. The resultant shell is removed and constitutes the mold. The molds are generally supported by frameworks to avoid distortion of the walls of the mold from the weight of the applied laminate. This is particularly true during the injection and curing of the foam between the shells because of expansion forces generated during curing of the foam. This is the same technique used to construct molds for boats and other structures which comprise compound curved surfaces. While the technique works well for such compound curved structures, equipment shelters generally comprise a plurality of large rectangular flat surfaces. In creating a mold which comprises such large flat surfaces from a pattern, it is not unusual for the laminate to partially separate from the pattern surface prior to complete curing. This generally requires the entire process to be repeated. Furthermore, it is common for bubbles and other disturbances to occur even in a properly cured mold, producing deformities in the mold. The resultant defects in the molded product are particularly noticeable in the multitude of large flat surfaces which constitute a shelter. In addition, the construction of such full scale patterns themselves is an expensive and time consuming process.
A need exists for improved shelter shell molds and a method of producing such improved molds. The molds should be relatively deformity free, inexpensive and durable enough to last for production runs of considerable quantities of shelters. The method of making the molds should eliminate the need for expensive and time-consuming preparation of a full scale pattern of the shells to be molded, and should eliminate separation problems and other defects common in molding large flat walls.